1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device, and more particularly, an organic electroluminescent device which has a structure for preventing damage or denaturalization caused by external circumstance such as moisture.
2. Description of the Related Art
In recent years, flat panel displays have been noticed, which overcomes weight and size problems of the cathode ray tube (CRT) to have small size and light weight characteristics. Such flat panel displays include liquid crystal displays (LCD), organic electroluminescent displays (OELD), field emitter displays (FED), plasma display panels (PDP), or the like.
The organic EL device among these displays has advantages such as wider temperature range, higher impact or vibration resistance, wider viewing angle, and faster response time for providing a clear moving picture compared to other flat panel displays, so that it has been noticed as a next-generation flat panel display.
The organic EL device may be classified into a passive matrix type and an active matrix type according to its driving method. The basic structure of the organic EL device includes a substrate 10, a pixel portion 18, which is comprised of a first electrode layer 12 on the substrate 10, an organic layer 14 having at least one emission layer on the first electrode layer 12, a second layer 16 on the organic layer 14, and a passivation layer (not shown) on the second electrode layer 16, as shown in FIG. 1.
The pixel portion 18 formed on the substrate 10 is encapsulated by a sealing cap 20 to prevent its performance and lifetime from being degraded due to external circumstance such as moisture.
The sealing cap 20 is made of metal material to be configured to have an internal space, and is adhered to the substrate 10 by means of adhesive 22 applied along the circumference of the substrate 10.
A moisture absorbent 24 is arranged within the internal space of the sealing cap 20 to prevent luminous efficiency and emission area of the pixel portion 18 from being degraded due to moisture or the like.
In addition, a method for encapsulating the organic EL device with the pixel portion formed on the substrate 10, comprises the step of preparing a sealing cap 20 made of a metal material having a predetermined internal space, and the substrate 10 as shown in FIG. 2.
Next, an adhesive 22 is applied in a thickness of 150 μm along the circumference of the sealing cap 20 in the S4 step.
After the substrate 10 is positioned on the sealing cap 20 and applied with the adhesive 22, a predetermined pressure is applied to the substrate 10 or the sealing cap 20 to reduce the thickness of the adhesive 22 to be about 50 μm, which leads to attach the sealing cap 20 to the substrate 10 in the S6 step. In this case, since the thickness of the adhesive 22 is decreased by the pressure applied to the substrate 10 or the sealing cap 20, the internal space that has been formed by the substrate 10 and the sealing cap 20 becomes reduced, which causes the internal pressure of the internal space to be 1.1 to 3 times the pressure before it is applied to.
The adhesive 22 between the substrate 10 and the sealing cap 20 is subject to ultra violet (UV) curing at a predetermined temperature in the S8 step. In this case, the moisture present within the internal space formed by the substrate 10 and the sealing cap 20 is removed by the moisture absorbent 24, which leads to prevent performance of the pixel portion 18 from being degraded due to the moisture or the like.
However, in the organic EL device having the above-mentioned structure, some of the adhesive applied on the substrate is come off or the width of the applied adhesive is decreased, and curing defects occur due to the thickness difference of the adhesive, which results in moisture infiltration into the defective portion of the adhesive to thereby deteriorate the organic EL device.
To solve the above-mentioned problems, there has been an attempt to have a sealing groove for accommodating the adhesive to be applied on the substrate 10 or the sealing cap 20. As a result, the thickness of the adhesive to be applied becomes constant and the adhesive strength is increased to thereby reduce the adhesive defects to some extent.
However, when the UV curing is performed for the adhesive, UV is blocked by the metal wiring of the substrate so that sufficient amount of UV is not irradiated to the adhesive, which causes the insufficient hardness of the adhesive. In addition, the remaining gas is emitted after the curing is performed, and causes the adhesive strength to be degraded as time goes. Due to these defects, impurity, oxygen, or moisture is penetrated from external to cause the moisture absorbent function to be degraded, which still causes the lifetime of the organic EL device to be shortened.